A published Japanese Patent Application, Kokai No. 2005-183375 shows a solid electrolyte type fuel cell system including a cell stack assembly composed of a plurality of cell stacks, a gas manifold provided with the cell stack assembly, a reformer to produce a hydrogen-rich reformed gas by steam reforming, and a reformed gas supplying tube connecting the reformer to the gas manifold, which are provided inside a housing. This fuel cell system is arranged to heat the reformer with an off gas discharged from the cell stack assembly.
There are three different reforming reactions performed in the reformer. Steam reforming reaction is endothermic, and partial oxidation reaction is exothermic. The third type is auto thermal reaction combining the steam reforming reaction and partial oxidation reaction, to balance the heat absorbed and the heat released. The fuel cell system of the above-mentioned patent document enables efficient operation by supplying waste heat from the cell stack assembly efficiently to the reformer, and thereby increasing the percentage of the endothermic steam reformation.